Fluorocarbon elastomers are synthetic elastomeric polymers with a high fluorine content--see, for example, W. M. Grootaert, G. H. Millet, & A. T. Worm. Fluorocarbon Elastomers, 8 Kirk-Othmer Encyclopedia Of Chemical Technology 990-1005 (4th ed. 1993). Fluorocarbon elastomers, particularly the copolymers of vinylidene fluoride with other ethylenically unsaturated halogenated monomers such as hexafluoropropene (C.sub.3 F.sub.6) have become the polymers of choice for high temperature applications, such as seals, gaskets, and linings. These polymers exhibit favorable properties against the exposure to aggressive environments such as solvents, lubricants, and oxidizing or reducing agents. Additionally, these polymers can be compounded and cured to have high tensile strength, good tear resistance, and low compression set.
Presently used curing agents for fluoroelastomers include aromatic polyhydroxy crosslinking agents, such as polyphenols, used in combination with certain organo-onium vulcanization accelerators. U.S. Pat. No. 4,882,390 (Grootaert et al.), U.S. Pat. No. 4,912,171 (Grootaert et al.) and U.S. Pat. No. 5,086,123 (Guenthner et al.), for example, describe these compounds.
Certain classes of low glass transition temperature (T.sub.g) fluoroelastomers are conventionally formed by curing fluoroelastomer compositions with certain functional fluoroaliphatic mono- and polyethers such as described in U.S. Pat. Nos. 5,266,650 and 5,384,374, both to Guerra et al. The polyethers described by these patents may be used in combination with one or more of the above-referenced crosslinking agents, or the polyethers taught by the references may be used themselves to crosslink the fluoroelastomer compositions.
In accordance with conventional curing processes, desired amounts of compounding ingredients and other conventional adjuvants or ingredients are added to the unvulcanized fluorocarbon elastomer stock and intimately admixed or compounded therewith by employing any of the usual rubber mixing devices such as Banbury mixers, roll mills, or any other convenient mixing device. The temperature of the ingredients during mixing typically will not rise above 120.degree. C. During mixing, the components and adjuvants are distributed throughout the fluorocarbon gum. The curing process typically comprises either extrusion of the compounded mixture into a mold (injection molding) or pressing of the compounded mixture in a mold (press cure), e.g. a cavity or a transfer mold, followed subsequently by oven-curing (post cure). Injection molding of the compounded mixture is usually conducted in two temperature ranges. The compounded mixture is first introduced into an injection barrel whose orifice is typically operated at about 70.degree. C. to 90.degree. C. The mixture is then forced down the extruder barrel with the aid of a mechanical screw. The mold into which the mixture is extruded is typically operated at about 180.degree. C. to 200.degree. C. The resulting molded articles are also typically ultimately subjected to a post-cure at elevated temperatures for an extended period of time, e.g. at temperatures above 200.degree. C. for 16 to 24 hours.
A drawback to the production of many of these conventional fluoroelastomers is their tendency towards "scorching," i.e., the premature or excessively rapid cure of the compounded composition when exposed to elevated temperatures or high shear conditions that produce a high temperature environment. This scorching behavior is particularly troublesome for injection-molded fluoroelastomers, wherein scorching is characterized by a premature cure initiation occurring prior to and during injection of the compounded composition into a mold. The point of cure initiation for injection-molded fluoroelastomers may be defined as the time after which the compounded fluoroelastomer is subjected to injection molding temperature conditions (i.e., upon introduction into an injection barrel at a temperature above approximately 70.degree. C.) when the curing compound begins to gel or harden. Such a change in physical properties, particularly the corresponding viscosity increase, can greatly reduce processing efficiency by hindering the ability to inject the compounded mixture into a mold.